The Jim DeMint-founded Senate Conservatives Fund is targeting Sen. Jeff Flake (R-AZ) for not willing to endorse an effort to defund Obamacare or shut down the federal government in a new radio ad airing Monday.

"Senator Jeff Flake is no where to be found," says the narrator in the ad. "Jeff Flake won't stand up to President Obama and join conservatives in pledging to oppose funding for the implementation of Obamacare. So what's his excuse?"

Speaking at a town hall in Gilbert, Ariz. this weekend, Flake said that while he "would love" to defund the law, "the continuing resolution is not the mechanism" to do so.